Technology is known in which a bonded substrate, in which the polished surfaces (the bonding surfaces) of two mirror-polished wafers are bonded to each other, is formed as a wafer for a high performance device that uses a TSV process (Through Silicon Via), etc. The two wafers are bonded after performing processing such as forming an oxide layer on at least one wafer of the two wafers by plasma processing, etc. After the bonding, after increasing the bonding strength by performing heat treatment as necessary, the thickness of the bonded wafer is reduced to the desired thickness by grinding and polishing. According to such technology, the bonding to each other of substrates typified by wafers can be performed without interposing a bonding agent, etc., between the substrates. Therefore, diversification of the process conditions in the processing (high-temperature processing and chemical processing) after the bonding can be realized.
Normally, in such a bonding apparatus, the bonding is performed while pushing one substrate to be bonded in the direction of the other substrate and causing the one substrate to deform.
As recited in Patent Literature 1, a first substrate which is one substrate to be bonded is held on a stage; and a second substrate which is another substrate is opposingly arranged with a spacing between the second substrate and the first substrate in a state of the peripheral portion of the second substrate being held; and the bonding is performed while pushing, from above, one prescribed point of the surface of the second substrate on the side opposite to the bonding surface.
Also, as recited in Patent Literature 2, one semiconductor wafer to be bonded is caused to contact another semiconductor wafer in a state in which the one semiconductor wafer is held by vacuum suction to a rubber chuck and warped so that the bonding surface central portion of the one semiconductor wafer has a protruding shape; and subsequently, the bonding of the two wafers is performed by relaxing the warp of the one semiconductor wafer by introducing air, etc., into the rubber chuck interior.
However, as in Patent Literature 1, the substrate may deflect due to its own weight in the case where the circumferential edge portion of the second substrate is held. Further, in the case where the bonding is performed while pushing the second substrate from above, the deflection of the second substrate becomes large; and strain occurs in the second substrate.
Moreover, as in Patent Literature 2, even in the case where the bonding is performed by deforming the wafer in a protruding configuration to conform to the placement surface of the rubber chuck, there is a risk that strain may occur in the wafer.
In the case where such strain occurs in the bonded substrates, the alignment of the two substrates cannot be performed appropriately; and reliable bonding without shifting is difficult.